Coupled ring oscillator

ABSTRACT

A circuit including a first oscillator configured to oscillate at a first frequency; a second oscillator configured to oscillate at a second frequency, the second frequency being different from and one of a harmonic or sub-harmonic of the first frequency; and a coupling between the first oscillator and the second oscillator configured to injection lock at least one of the first oscillator and second oscillator to the other of the first oscillator and second oscillator.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the priority benefit of Indian patentapplication number 3127/Del/2010, filed on Dec. 28, 2010, entitledCOUPLED RING OSCILLATOR, which is hereby incorporated by reference tothe maximum extent allowable by law.

BACKGROUND OF THE INVENTION

1. Field on the Invention

The present invention relates to coupled ring oscillators, for examplebut not exclusively to coupled ring oscillators in integrated circuits.

2. Discussion of the Related Art

Oscillators are commonly used in radio frequency communication systemsand many other applications, for example, to perform mixing or frequencytranslation of information signals and channel selection. Oscillatorsare generally also present in all digital electrical systems whichrequire a time reference, for example a clock signal, in order tosynchronize operations. The ideal oscillator would be one providing aperfect time reference however typical physical practical oscillatorsproduce signals which contain undesired noise in the form of eitheramplitude or phase noise.

The performance of ring oscillators, a cascaded combination of delaystages connected in a closed loop have been of great interest to circuitdesigners for implementation as oscillators because of their usefulfeatures such as being easily implementable, being able to achieveoscillations with low voltages, providing high frequency oscillationswith low power, being electrically tuneable over a wide tuning range andbeing able to be tuned whilst also providing a multi-phase output. Theoscillation frequency of a ring oscillator depends on the propagationdelay T_(d) per stage and the number of stages used in the ringstructure M. In order to achieve self-sustaining oscillations, the ringprovides a phase shift where each stage provides a phase shift of π/Mfor a M stage ring oscillator and an inversion provides the remainingphase shift of π. Ring oscillators have an inherent problem in that thefaster a ring oscillator is to operate with a fixed delay stage thefewer the number of delay stages are required.

To overcome this problem coupled ring oscillators are known wherein twoor more ring oscillators are coupled together in such a way that thesystem oscillates with an effect between the two oscillators. However inconventional coupled ring oscillators operating at low frequencies thecompromise between phase noise or power consumption to reduce the phasenoise can be problematic in oscillator design.

SUMMARY OF THE INVENTION

It is an aim of embodiments of the present application to overcome atleast one of these problems.

According to a first aspect, there is provided a circuit comprising: afirst oscillator configured to oscillate at a first frequency; a secondoscillator configured to oscillate at a second frequency, the secondfrequency being different from and one of a harmonic or sub-harmonic ofthe first frequency; and a coupling between the first oscillator and thesecond oscillator configured to injection lock at least one of the firstoscillator and second oscillator to the other of the first oscillatorand second oscillator.

The coupling may comprise at least one of: a unilateral coupling fromthe first oscillator to the second oscillator; a unilateral couplingfrom the second oscillator to the first oscillator; and a bilateralcoupling between the first oscillator and the second oscillator.

The coupling may comprise at least one of: a filter; a direct coupling;a level shifter; a diode; and an impulse generator.

The filter may comprise at least one of: a low pass filter configured tocouple from a higher frequency oscillator to a lower frequencyoscillator; a high pass filter configured to couple from a lowerfrequency oscillator to a higher frequency oscillator.

The first oscillator may comprise at least two oscillators.

At least one of the first oscillator of the at least two oscillators maycomprise a crystal oscillator.

At least two oscillators from the first oscillator of the at least twooscillators may be homogeneously coupled such as to produce a low noisefirst frequency.

At least one oscillator from the first oscillator of the at least twooscillators may be configured to oscillate of the at a first frequency;and a second at least one oscillator from the first oscillator at leasttwo oscillators may be configured to oscillate at a third frequencywherein the at least one oscillator from the first oscillator of the atleast two oscillators and the second at least one oscillator from thefirst oscillator of the at least two oscillators may be heterogeneouslycoupled.

The first oscillator may comprise at least one of: a ring oscillator; anLC oscillator; a crystal oscillator; and a delay line oscillator.

The second oscillator may comprise at least one of: a ring oscillator;an LC oscillator; a crystal oscillator; and a delay line oscillator.

An integrated circuit may comprise a circuit as described herein.

A mixer may comprise a circuit as described herein.

A phase locked loop (PLL) may comprise a circuit as described herein.

An analog to digital converter (ADC) may comprise a circuit as describedherein.

A communications device may comprise a circuit as described herein.

According to a second aspect, there is provided a method comprising:oscillating a first oscillator at a first frequency; oscillating asecond oscillator at a second frequency, the second frequency beingdifferent from and one of a harmonic or sub-harmonic of the firstfrequency; and injection lock coupling between the first oscillator andthe second oscillator.

Coupling may comprise performing at least one of: unilateral couplingfrom the first oscillator to the second oscillator; unilateral couplingfrom the second oscillator to the first oscillator; and bilateralcoupling between the first oscillator and the second oscillator.

Coupling may comprise at least one of: filtering at least one output ofthe first oscillator and second oscillator; direct coupling at least oneoutput of the first oscillator and the second oscillator; level shiftingat least one output of the first oscillator and the second oscillator;diode coupling at least one output of the first oscillator and thesecond oscillator; and impulse generating at least one output of thefirst oscillator and the second oscillator.

Filtering may comprise performing at least one of: low pass filteringfrom the oscillator operating at a higher frequency to the oscillatoroperating at a lower frequency; and high pass filtering from theoscillator operating at a lower frequency to the oscillator operating ata higher frequency.

Oscillating a first oscillator at a first frequency may compriseoscillating at least two oscillators.

At least one of the first oscillator of the at least two oscillators maycomprise a crystal oscillator.

Oscillating the at least two oscillators from the first oscillator maycomprise homogeneously coupling the at least two oscillators such as toproduce a low noise first frequency.

Oscillating at least two oscillators may comprise: oscillating at leastone oscillator from the first oscillator of the at least two oscillatorsat the first frequency; oscillating a second at least one oscillatorfrom the first oscillator of the at least two oscillators at a thirdfrequency, the third frequency being different and one of a harmonic orsub-harmonic of the first frequency; and heterogeneously coupling the atleast one oscillator from the first oscillator of the at least twooscillators and the second at least one oscillator from the firstoscillator of the at least two oscillators.

The first oscillator may comprise at least one of: a ring oscillator; aLC oscillator; a crystal oscillator; and a delay line oscillator.

The second oscillator may comprise at least one of: a ring oscillator; aLC oscillator; a crystal oscillator; and a delay line oscillator.

According to a third aspect there is provided a computer-readable mediumencoded with instructions that, when executed by a computer, perform:oscillating a first oscillator at a first frequency; oscillating asecond oscillator at a second frequency, the second frequency beingdifferent from and one of a harmonic or sub-harmonic of the firstfrequency; and injection lock coupling between the first oscillator andthe second oscillator.

Coupling may cause the computer to perform at least one of: unilateralcoupling from the first oscillator to the second oscillator; unilateralcoupling from the second oscillator to the first oscillator; andbilateral coupling between the first oscillator and the secondoscillator.

Coupling may cause the computer to perform at least one of: filtering atleast one output of the first oscillator and second oscillator; directcoupling at least one output of the first oscillator and secondoscillator; level shifting at least one output of the first oscillatorand second oscillator; diode coupling at least one output of the firstoscillator and second oscillator; and impulse generating at least oneoutput of the first oscillator and second oscillator.

Filtering may cause the computer to perform at least one of: low passfiltering from the oscillator operating at a higher frequency to theoscillator operating at a lower frequency; and high pass filtering fromthe oscillator operating at a lower frequency to the oscillatoroperating at a higher frequency.

Oscillating a first oscillator at a first frequency may cause thecomputer to perform oscillating at least two oscillators.

At least one of the first oscillator at least two oscillators maycomprise a crystal oscillator.

Oscillating the at least two oscillators from the first oscillator maycause the computer to perform homogeneously coupling the at leastoscillators such as to produce a low noise first frequency.

Oscillating at least two oscillators may cause the computer to perform:oscillating at least one oscillator from the first oscillator at leasttwo oscillators at the first frequency; oscillating a second at leastone oscillator from the first oscillator at least two oscillators at athird frequency, the third frequency being different and one of aharmonic or sub-harmonic of to the first frequency; and heterogeneouslycoupling the at least one oscillator from the first oscillator of the atleast two oscillators and the second at least one oscillator from thefirst oscillator of the at least two oscillators.

The first oscillator may comprise at least one of: a ring oscillator; anLC oscillator; a crystal oscillator; and a delay line oscillator.

The second oscillator may comprise at least one of: a ring oscillator;an LC oscillator; a crystal oscillator; and a delay line oscillator.

According to a fourth aspect there is provided an apparatus comprisingat least one processor and at least one memory including computerprogram code in the at least one memory and the computer program codeconfigured to, with the at least one processor, cause the apparatus atleast to perform: oscillating a first oscillator at a first frequency;oscillating a second oscillator at a second frequency, the secondfrequency being different from and one of a harmonic or sub-harmonic ofthe first frequency; and injection lock coupling between the firstoscillator and the second oscillator.

Coupling may cause the apparatus to perform performing at least one of:unilateral coupling from the first oscillator to the second oscillator;unilateral coupling from the second oscillator to the first oscillator;and bilateral coupling between the first oscillator and the secondoscillator.

Coupling may cause the apparatus to perform at least one of: filteringat least one output of the first oscillator and second oscillator;direct coupling at least one output of the first oscillator and thesecond oscillator; level shifting at least one output of the firstoscillator and the second oscillator; diode coupling at least one outputof the first oscillator and the second oscillator; and impulsegenerating at least one output of the first oscillator and the secondoscillator.

Filtering may cause the apparatus to perform at least one of: low passfiltering from the oscillator operating at a higher frequency to theoscillator operating at a lower frequency; and high pass filtering fromthe oscillator operating at a lower frequency to the oscillatoroperating at a higher frequency.

Oscillating a first oscillator at a first frequency may cause theapparatus to perform oscillating at least two oscillators.

At least one of the first oscillator at least two oscillators maycomprise a crystal oscillator.

Oscillating the at least two oscillators from the first oscillator maycause the apparatus to perform homogeneously coupling the at least twooscillators such as to produce a low noise first frequency.

Oscillating at least two oscillators may cause the apparatus to perform:oscillating at least one oscillator from the first oscillator of the atleast two oscillators at the first frequency; oscillating a second atleast one oscillator from the first oscillator of the at least twooscillators at a third frequency, the third frequency being differentand one of a harmonic or sub-harmonic of the first frequency; andheterogeneously coupling the at least one oscillator from the firstoscillator of the at least two oscillators and the second at least oneoscillator from the first oscillator of the at least two oscillators.

The first oscillator may comprise at least one of: a ring oscillator; anLC oscillator; a crystal oscillator; and a delay line oscillator.

The second oscillator may comprise at least one of: a ring oscillator;an LC oscillator; a crystal oscillator; and a delay line oscillator.

According to a fifth aspect there is provided an apparatus comprising:means for oscillating at a first frequency; means for oscillating at asecond frequency, the second frequency being different from and one of aharmonic or sub-harmonic of the first frequency; and means for injectionlock coupling between the means for oscillating at a first frequency andthe means for oscillating at a second frequency.

The means for injection lock coupling may comprise at least one of:means for unilateral coupling from the means for oscillating at a firstfrequency to the means for oscillating at a second frequency; means forunilateral coupling from the means for oscillating at a second frequencyto the means for oscillating at a first frequency; and means forbilateral coupling between the means for oscillating at a firstfrequency and the means for oscillating at a second frequency.

The means for coupling may comprise at least one of: means forfiltering; means for direct coupling; means for level shifting; meansfor diode coupling; and means for impulse generating.

Means for filtering may comprise: low pass filtering from the oscillatoroperating at a higher frequency to the oscillator operating at a lowerfrequency; and high pass filtering from the oscillator operating at alower frequency to the oscillator operating at a higher frequency.

Means for oscillating at a first frequency may comprise at least twomeans for oscillating.

At least one of the at least two means for oscillating may comprise acrystal oscillator.

The at least two means for oscillating may comprise means forhomogeneously coupling the at least two oscillators such as to produce alow noise first frequency.

The at least two means for oscillating may comprise: at least one meansfor oscillating at the first frequency; at least one means foroscillating at a third frequency, the third frequency being differentand one of a harmonic or sub-harmonic of the first frequency; and meansfor heterogeneously coupling the at least one means for oscillating atthe first frequency and the at least one means for oscillating at athird frequency.

The means for oscillating at a first frequency may comprise at least oneof: a ring oscillator; an LC oscillator; a crystal oscillator; and adelay line oscillator.

The means for oscillating at a second frequency may comprise at leastone of: a ring oscillator; an LC oscillator; a crystal oscillator; and adelay line oscillator.

BRIEF DESCRIPTION OF THE DRAWINGS

Reference will be made by way of example only to the accompanyingdrawings in which:

FIG. 1 shows schematically an overview of a coupled ring oscillatordevice according to embodiments of the application;

FIG. 2 shows schematically the first ring oscillator, inverter and highpass filter circuits as shown in FIG. 1 according to some embodiments ofthe application;

FIG. 3 shows schematically the second ring oscillator as shown in FIG. 1according to some embodiments of the application; and

FIG. 4 shows schematically a graph of the improvements of a simulatedcoupled ring oscillator device according to some embodiments of theapplication.

DETAILED DESCRIPTION

This document describes apparatus and methods for operating coupled ringoscillators. The embodiments of the application aim to produce animprovement over present approaches.

With respect to FIG. 1, a coupled ring oscillator device is shownaccording to some embodiments of the application. The coupled ringoscillator device comprises a first ring oscillator 1, a second ringoscillator circuit ring oscillator 11, and a coupling arrangementconfigured to couple the first ring oscillator 1 to the second ringoscillator in a unilateral coupling. In other words, the couplingarrangement is a unidirectional coupling from the first ring oscillatorcircuit 1 to the second ring oscillator circuit 11. The coupling betweenthe first ring oscillator 1 and the second ring oscillator 11 is aheterogeneous coupling in that the first ring oscillator is configuredto oscillate at a frequency which is a harmonic or sub-harmonic of thesecond ring oscillator 11.

In some embodiments, the coupling arrangement configured to couple thefirst ring oscillator circuit 1 to the second ring oscillator circuit 11can comprise an inverter 5 and a high pass filter 3. However, anysuitable coupling arrangement can be used. For example, a low passfilter in one direction and a high pass filter in the other directioncan be implemented in some embodiments can be used to form a bi-lateralcoupling. Furthermore in some embodiments a unilateral couplingarrangement from a high frequency oscillator to a low frequencysub-harmonic frequency oscillator can comprise a low pass filter.

The inverter 5 can be implemented using any suitable device and beconfigured to implement a π phase shift on the output of the first ringoscillator circuit 1 at any suitable point. The inverter 5 canfurthermore in some embodiments be coupled to the high pass filtercircuit 3.

The high pass filter circuit 3 can in some embodiments be configured toreceive the output of the inverter as the input to be filtered andoutput a high pass filtered signal to the second ring oscillator circuit11. In some embodiment the high pass circuit 3 is configured to removeor compensate for dc and low frequency noise and contamination from thefirst ring circuit 1. The high pass filter circuit 3 can comprise, asshown in FIG. 1, a capacitor (C) 7 and a resistor or load (R) 9 coupledin a high pass arrangement wherein the output of the inverter 5 iscoupled to a first terminal of the capacitor 7 the second terminal ofthe capacitor 7 is coupled to the second ring oscillator circuit 11 andto the first terminal of the load 9 and the second terminal of the load9 is coupled to a dc reference voltage (in the example shown in FIG. 1the second terminal of the load 9 is coupled to the ground terminal).

In embodiments of the application, the first ring oscillator circuit 1can be configured to have at least one ring oscillator which isconfigured to operate at a harmonic period with respect to the secondring oscillator circuit 11. In other words, the first ring oscillatorcircuit 1 is configured to operate at a first frequency and the secondring oscillator circuit 11 configured to operate at a frequency which isa harmonic of the first frequency.

With respect to FIG. 2, the first ring oscillator circuit 1 according tosome embodiments of the application is shown in further detail. Thefirst ring oscillator 1 in such embodiments comprises a series of closecoupled inverters 101. The example shown in FIG. 2, for example, shows aseries of nineteen stages of inverters 101 wherein the output of aninverter is coupled to the input of a successive inverter and the outputof the last inverter is coupled to the input of the first inverter toclose the loop. The inverter 101 within the first ring oscillator can beimplemented in any suitable technology including a differential inverterarrangement.

The first ring oscillator circuit 1 furthermore is configured to have anoutput 103 which can be the output of any one of the inverters 101. Theoutput 103 of the first ring oscillator 1 as described herein is coupledto the input of the inverter 5.

The inverter 5 can be any suitable technology and output an invertedfirst ring oscillator circuit 1 signal to the input of the high passfilter 3.

The high pass filter 3 as described here comprises a capacitor. Thecapacitor can as shown in FIG. 2 be implemented as a transistor 7configured to operate as a capacitor. Furthermore the high pass filtercomprising a load can be implemented as a load coupled pair oftransistors 9. The output of the high pass filter INJ, the junctionbetween the capacitor and load is configured to be coupled to the secondring oscillator circuit 11.

With respect to FIG. 3, an example of the second ring oscillator circuit11 according to some embodiments of the application is shown. The secondring oscillator 11 comprises a series of three inverters arranged in aclose coupled configuration. In other words, the output of the lastinverter is coupled to the input of the first inverter.

The first inverter 205 is configured in these embodiments to receive notonly the inputs from the third inverter 203 but also the filteredinverted first ring oscillator circuit input. The first invertercomprises, in some embodiments, a first transistor 205 ₂ a PMOStransistor, which has a source node coupled to the high referencevoltage (V_(dd)), a drain node coupled to the source of a secondtransistor 205 ₁ and a gate node configured to receive the output of thethird inverter. The first inverter 205 further comprises a secondtransistor 205 ₁, a PMOS transistor, which has a source node coupled tothe drain node of the first transistor, a drain node coupled to thesource node of the third transistor 205 ₄, and a gate coupled to thefiltered inverted first ring oscillator circuit input INJ, the injectionlocking input. The first inverter also comprises a third transistor 205₄, a NMOS transistor, configured to have a source node coupled to thedrain node of the second transistor 205 ₁, a drain node coupled to thesource node of a fourth transistor 205 ₃ and a gate node coupled to ahigh reference voltage and therefore switching the third transistor‘on’. The first inverter 205 further comprises a fourth transistor 205₃, a NMOS transistor, configured with a source node coupled to the drainof the third transistor 205 ₄, a drain node coupled to a low referencevoltage (GND), and the gate node coupled to the output of the thirdinverter. The output of the first inverter 205 is taken from thejunction between the second transistor drain and the third transistorsource nodes.

The second inverter 201 and third inverter 203 each comprise a pair oftransistors arranged in a standard inverter configuration such that theinput to each inverter is coupled to the gate node of a first transistor201 ₁ 203 ₁, a PMOS transistor, and a second transistor 201 ₂ 203 ₂, aNMOS transistor, the source of the PMOS transistor is coupled to thehigh reference voltage, the drain of the PMOS connected to the source ofthe NMOS transistor, the drain of the NMOS transistor coupled to the lowreference voltage, and the output taken from the NMOS source/PMOS drainoutput.

In such embodiments, the first inverter 205 can, at the gate of thesecond transistor 205 ₁ receive the input from the output of the highpass filter via the injection locking input INJ. In other words, thecoupling between the first ring circuit and the second ring circuit inthe example shown is both one or unidirectional and heterogeneous inthat the first ring oscillator has a oscillation frequency which is asub-harmonic of the second ring oscillator.

Furthermore, as the second ring oscillator circuit 11 is configuredoperate at a higher frequency than the lower frequency oscillator, thefirst ring oscillator circuit 1 the harmonic coupling produces animproved noise output for the second ring oscillator circuit 11 than aconventional single ring oscillator but with a power consumption whichis less than a conventional homogeneous coupled oscillator system. Theperformance improvement can be seen as being a noise improvementgenerated due to the injection locking whereby the first oscillator ringproduces a signal which locks or stabilizes the second oscillator ring.Furthermore, as in this example the first oscillator ring is operatingat a frequency lower than the second oscillator ring, the powerconsumption of the first oscillator ring is less than the secondoscillator ring operating at the higher frequency and therefore thecombination of the oscillator rings has a power consumption lower thantwo ring oscillators operating at the second oscillator ring frequencyas used in a conventional homogeneously coupled oscillator system.

Although the embodiments of the application described herein show theexample harmonic coupling produced by a 19 stage to 3 stage coupled ringoscillator device it would be to understood that in some furtherembodiments other harmonics can be selected to provide a suitablecoupling ring oscillator device. For example, coupling a 3 stageoscillator with a 21 stage oscillator using such high pass filter andinverter logic can provide an injection locked improvement in noise of20 dB (improvement in phase noise) with increased current of only 5-6×.

In examples of oscillators employing embodiments of the application lowfrequency phase noise can be very low such as −140 dBc/Hz at 1 MHz withan offset of 8 MHz.

Furthermore it would be understood that power consumption of circuitsemploying embodiments of the application can also be low. For examplethe circuit shown herein with respect to FIGS. 2 and 3 would beapproximately 40 μW.

Although a two oscillator harmonic heterogeneous coupling is shown inthe examples herein it would be understood that in some embodimentsheterogeneous harmonic couplings could be employed between more than twooscillators. Furthermore in some embodiments there can, for example, bea chain of linked oscillators wherein at least one of the chain ofcouplings is a heterogeneous coupling. Furthermore although theoscillators shown herein are close coupled inverters it would beunderstood that in some embodiments other suitable oscillators can beused.

Thus, for example, in some embodiments a first oscillator could be aconventional crystal oscillator which is coupled to a chain ofheterogeneously coupled ring oscillators in order to supply a range offrequencies at harmonics or sub-harmonics of the crystal oscillator.

Furthermore in some embodiments at least one of the oscillators formingthe chain of oscillators can itself comprise a cluster of linkedoscillators. For example, a first frequency could be generated by atleast two homogeneously coupled ring oscillators of which at least oneis coupled to a second (higher) frequency ring oscillator. In such anexample the first frequency is injection locked by the homogeneouscouplings between the first frequency oscillators and thus have a muchlower noise level than a single first frequency oscillator but thisreduced noise first frequency oscillation can injection lock the secondfrequency oscillator to produce a low noise second frequency signal butrequiring current and power at much lower levels than if there weremultiple second frequency oscillators homogeneously coupled.

As shown in FIG. 4 the improvement of signal noise against frequency isshown for embodiments of the application. Thus the graph shown in FIG. 4shows noise against frequency plots for a single ring oscillator withthree inversion stages 301, a four coupled oscillator from threeinversion stages 303, a single ring oscillator operating at highfrequencies to 6 GHz 305, and the example heterogeneous coupledoscillator 307 whereby the heterogeneous oscillator has a significantlylower noise plot for all frequencies for a current which is only 6× thesingle 3 stage ring example simulation.

In general, the various embodiments of the invention may be implementedin hardware or special purpose circuits, software, logic or anycombination thereof. For example, some aspects may be implemented inhardware, while other aspects may be implemented in firmware or softwarewhich may be executed by a controller, microprocessor or other computingdevice, although the invention is not limited thereto. While variousaspects of the invention may be illustrated and described as blockdiagrams, flow charts, or using some other pictorial representation, itis well understood that these blocks, apparatus, systems, techniques ormethods described herein may be implemented in, as non-limitingexamples, hardware, software, firmware, special purpose circuits orlogic, general purpose hardware or controller or other computingdevices, or some combination thereof.

The embodiments of this application can be implemented by computersoftware executable by a data processor, such as in the processorentity, or by hardware, or by a combination of software and hardware.Further in this regard it should be noted that any blocks of the logicflow as in the Figures may represent program steps, or interconnectedlogic circuits, blocks and functions, or a combination of program stepsand logic circuits, blocks and functions. The software may be stored onsuch physical media as memory chips, or memory blocks implemented withinthe processor, magnetic media such as hard disk or floppy disks, andoptical media such as for example DVD and the data variants thereof, CD.

The memory may be of any type suitable to the local technicalenvironment and may be implemented using any suitable data storagetechnology, such as semiconductor-based memory devices, magnetic memorydevices and systems, optical memory devices and systems, fixed memoryand removable memory. The data processors may be of any type suitable tothe local technical environment, and may include one or more of generalpurpose computers, special purpose computers, microprocessors, digitalsignal processors (DSPs), application specific integrated circuits(ASIC), gate level circuits and processors based on multi-core processorarchitecture, as non-limiting examples.

As used in this application, the term ‘circuitry’ can refer tohardware-only circuit implementations (such as implementations in onlyanalog and/or digital circuitry) and to combinations of circuits andsoftware (and/or firmware), such as: to a combination of processor(s) or(ii) to portions of processor(s)/software (including digital signalprocessor(s)), to software, and memory(ies) that work together to causean apparatus, such as a mobile phone or server, to perform variousfunctions and to circuits, such as a microprocessor(s) or a portion of amicroprocessor(s), that require software or firmware for operation, evenif the software or firmware is not physically present.

This definition of ‘circuitry’ applies to all uses of this term in thisapplication, including any claims As a further example, as used in thisapplication, the term ‘circuitry’ would also cover an implementation ofa processor (or multiple processors) or portion of a processor and its(or their) accompanying software and/or firmware.

Whilst this detailed description has set forth some embodiments of thepresent invention, the appended claims cover other embodiments of thepresent application which differ from the described embodimentsaccording to various modifications and improvements. Other applicationsand configurations may be apparent to the person skilled in the art.

What is claimed is:
 1. A circuit comprising: at least one first ringoscillator configured to oscillate at a first frequency; a second ringoscillator configured to oscillate at a second frequency, the secondfrequency being different from the first frequency and one of a harmonicor sub-harmonic of the first frequency; and a coupling between the atleast one first ring oscillator and the second ring oscillatorconfigured to injection lock at least one of the at least one first ringoscillator and the second ring oscillator to the other of the at leastone first ring oscillator and the second ring oscillator, wherein: thesecond ring oscillator comprises a plurality of inverters, an inverterfrom the plurality of inverters comprising at least two transistors; anda gate of a transistor of the at least two transistors is coupled to anoutput from the coupling via an injection locking input.
 2. The circuitas claimed in claim 1, wherein the coupling comprises at least one of: aunilateral coupling from the at least one first ring oscillator to thesecond ring oscillator; a unilateral coupling from the second ringoscillator to the at least one first ring oscillator; and a bilateralcoupling between the at least one first ring oscillator and the secondring oscillator.
 3. The circuit as claimed in claim 1, wherein thecoupling comprises at least one of: a filter; a direct coupling; a levelshifter; a diode; and an impulse generator.
 4. The circuit as claimed inclaim 3, wherein the filter comprises at least one of: a low pass filterconfigured to couple from a higher frequency ring oscillator to a lowerfrequency ring oscillator; a high pass filter configured to couple froma lower frequency ring oscillator to a higher frequency ring oscillator.5. The circuit as claimed in claim 1, wherein the at least one firstring oscillator comprises at least two oscillators.
 6. The circuit asclaimed in claim 5, wherein at least two oscillators from the at leasttwo oscillators are homogeneously coupled such as to produce a low noisefirst frequency.
 7. The circuit as claimed in claim 5, wherein: at leastone oscillator from the at least two oscillators is configured tooscillate at a first frequency; a second at least one oscillator fromthe at least two oscillators is configured to oscillate at a thirdfrequency; and the at least one oscillator from the at least twooscillators and the second at least one oscillator from the at least twooscillators are heterogeneously coupled.
 8. An integrated circuitcomprising a circuit as claimed in claim
 1. 9. A mixer comprising acircuit as claimed in claim
 1. 10. A phase locked loop (PLL) comprisinga circuit as claimed in claim
 1. 11. An analog to digital converter(ADC) comprising a circuit as claimed in claim
 1. 12. A communicationsdevice comprising a circuit as claimed in claim
 1. 13. A methodcomprising: oscillating at least one first ring oscillator at a firstfrequency; oscillating a second ring oscillator at a second frequency,the second frequency being different from the first frequency and one ofa harmonic or sub-harmonic of the first frequency; and injection lockcoupling between the at least one first ring oscillator and the secondring oscillator to form an injection lock coupling, wherein: the secondring oscillator comprises a plurality of inverters, an inverter from theplurality of inverters comprising at least two transistors; and themethod further comprises coupling a gate of a transistor of the at leasttwo transistors to an output from the injection lock coupling via aninjection locking input.
 14. The method as claimed in claim 13, whereincoupling comprises performing at least one of: unilateral coupling fromthe at least one first ring oscillator to the second ring oscillator;unilateral coupling from the second ring oscillator to the at least onefirst ring oscillator; and bilateral coupling between the at least onefirst ring oscillator and the second ring oscillator.
 15. The method asclaimed in claim 13, wherein coupling comprises at least one of:filtering at least one output of the at least one first ring oscillatorand the second ring oscillator; direct coupling at least one output ofthe at least one first ring oscillator and the second ring oscillator;level shifting at least one output of the at least one first ringoscillator and the second ring oscillator; diode coupling at least oneoutput of the at least one first ring oscillator and the second ringoscillator; and impulse generating at least one output of the at leastone first ring oscillator and the second ring oscillator.
 16. The methodas claimed in claim 15, wherein filtering comprises performing at leastone of: low pass filtering from a ring oscillator operating at a higherfrequency to a ring oscillator operating at a lower frequency; high passfiltering from the ring oscillator operating at a lower frequency to thering oscillator operating at a higher frequency.
 17. The method asclaimed in claim 13, wherein the oscillating the at least one first ringoscillator at a first frequency comprises oscillating at least twooscillators.
 18. The method as claimed in claim 17, wherein oscillatingthe at least two oscillators from the at least one first ring oscillatorcomprises homogeneously coupling the at least two oscillators such as toproduce a low noise first frequency.
 19. The method as claimed in claim17, wherein oscillating at least two oscillators comprises oscillatingat least one oscillator from the first ring oscillator of the at leasttwo oscillators at the first frequency; and oscillating a second atleast one oscillator from the first ring oscillator of the at least twooscillators at a third frequency, the third frequency being differentand one of a harmonic or sub-harmonic of the first frequency; andheterogeneously coupling the at least one oscillator from the first ringoscillator of the at least two oscillators and the second at least oneoscillator from the first ring oscillator of the at least twooscillators.